This invention relates to ammonium nitrate and more particularly to crystalline growth inhibitor for ammonium nitrate.
Ammonium nitrate (AN) is used in solid propellant, gas generators and explosives. A well recognized inherent property of ammonium nitrate is its dimensional instability when heated or cooled. When heated, AN experiences a .about.3% volumetric contraction at -18 to 0.degree. C. and a .about.3.5% expansion in volume at 32.degree. to 50.degree. C. Upon cooling, the reverse occurs but at different temperatures with a resultant net volume charge after thermal cycling. This expansion or contraction occurs in the temperature range of use of rocket motors or explosives. There have been catastrophic failures of rocket motors, as a result of excessive pressure caused by cracks and excess burn surface resulting from the undesirable expansion and contraction. A solution to this problem is badly needed.
The current solution to this dimensional instability problem is addition of 10 to 17% by weight of potassium nitrate to the AN. However, this approach results in a large amount of residue in the combustion products, and thus tends to corrode and plug the rocket nozzle. Furthermore, this added potassium nitrate lowers the weight percent of AN in the propellant and reduces the performance of the solid propellants and explosives in which it is used. For the above mentioned reasons, this approach is not satisfactory.
The present invention provides a solution to the above problem. By the addition of only 0.5 to 2% by weight of a specific crystal growth inhibitor, we have eliminated the undesirable abrupt expansion and contraction of ammonium nitrate in the temperature range of use of a rocket motor and explosives (-55.degree. C. to 80.degree. C.). Specifically, we have found that potassium fluoride (KF) added to NH.sub.4 NO.sub.3 in these small amounts effectively inhibits crystalline growth to the less dense orthothombic phase III of the 5 ammonium nitrate phases described below, and prevents the otherwise normal contraction during the transition from Phase V to Phase IV. The potassium fluoride must be added in the molten phase (I) and the KF modified AN then cooled.
This KF modified AN does not experience the otherwise normal excessive volume growth of up to 25% upon thermal cycling in the -55.degree. to 80.degree. C. temperature range, and yet does not produce excessive residues like KNO.sub.3 modified AN.
Thus, the KF significantly alters the normal inherent undesirable characteristics of NH.sub.4 NO.sub.3 while eliminating the undesirable characteristics of conventional KNO.sub.3 additive.
As used below, "DSC" means differential scanning calorimetry and "TMA" means thermo mechanical analysis (length expansion versus temperature).